Some new calendar documents created in Lotus Notes do not synchronize via wireless, while the other vast majority seems to be OK.
I have looked for the log and found the corresponding line beginning with "OTAC Sending ota update...", but I do not found the connecting entry: "Message sent to handheld..." and there are no error messages in the log regarding this calendar item.
Could you please advise what reasons/settings can prevent delivering to the handheld?

Have you performed the basic maintenance tasks on the affected user's mail files or on the \Lotus\Domino\Data\BES directory? SOmetimes the database may have corrupted views that prevent it from updating.
The typical tasks that you should run (either online or off-line) are fixup, updall and compact in that order.
You can also use the following keyboard shortcuts:
F9 - update the current views
SHIFT + F9 - rebuilds the current view
CTRL + SHIFT + F9 - rebuilds all views and updates all other views.